This invention generally relates to an apparatus and method for scanning a static document and more particularly to a facsimile scanner utilizing a pair of rotating spiral apertures in combination with a longitudinal slit for defining a scanning aperture.
Facsimile is the electrical transmission of a static replica of a document from one location to another. The document is divided, by optical means, into a large number of small discrete areas in the order of 10,000 or more per square inch. The reflective quality of each document area is either black, white, or a shade of grey, and is measured and represented by an electrical signal transmitted by conventional means. The quality of the reproduced document depends, to a great extent, upon the resolution of the scanning system; that is, the greater the number of discrete areas into which the document is divided during scanning, the higher is said to be the system resolution.
In a well-known facsimile scanning device, light is reflected sequentially from each minute document area as determined by the coincidence of two rotating spiral apertures and a longitudinal slit. The reflected light is in the form of a serial stream of light impulse levels with their intensity determined by the reflective qualities of the document areas. These photon pulses are passed through an optical system to a photomultiplier which converts the light level reflected from each discrete document area into electrical signals for transmission. One arrangement utilizing such coincident rotating spiral apertures is described in U.S. Pat. No 2,967,907 issued Jan. 10, 1961 to G. M. Stamps. As described in this reference, the discs defining the spiral apertures are rotated and their coincidence with a fixed longitudinal slit defines the scanning aperture which moves across the document along a linear path. After a select line of the document has been scanned, the optical system advances so that the next document line to be scanned is focused at the exposure slit. It will be appreciated that in such an arrangement each portion of the document lying along a line normal to the slit is exposed by the same sections of each of the spiral apertures and with the discs in the same relative positions. Due to the relatively small width of the spiral apertures, being in the order of 1/1,000 of an inch, the possibility of dust accumulating on the discs and blocking portions of the helical apertures is quite high. Any such obstruction which fully or partially blocks a portion of one of the spiral apertures will reoccur at the exposure aperture at the same relative locations on the document thereby producing an objectionable, visibly discernible streak across the reproduction. Prior scanners have attempted to overcome this problem by increasing the size of the sampling aperture in relation to the anticipated size of the obstructing particle so that the total area of the aperture is large in comparison to the size of the obstruction. One approach is to increase the size of the disc while maintaining the same number of spiral revolutions. However with such an approach, the disc becomes quite large, thus increasing the mounting and motor drive requirements. Further, the effect of a partial aperture blockage is integrated into the related sample during each scan cycle, thus producing a streak of incorrect grey scale. An alternative approach suggested has been to increase the size of the scanning aperture by increasing the number of spiral revolutions while maintaining the disc size constant. However, with this arrangement, the rotational speed of the disc must be considerably increased to maintain acceptable system resolution. Such a speed increase, of course, increases noise and vibration as well as raises the inertial stresses to which the rotating disc is subject consequently requiring greater drive power and more precise disc balance.
The illustrated embodiment materially overcomes the aforementioned problems while minimizing the adverse effects of dust and dirt contaminants on the spiral apertures by means of what may be termed "spiral aperture redundancy". That is, consecutive document scans are made by alternate spirals defined by a single disc and thus the adverse effects of a blockage of one of the spirals will not reoccur consecutively, but rather, will be spaced by a number of scan lines which is one less than the number of spirals defined by the disc. A viewer's eye will effectively integrate out such apparently random copy defects. The greater the number of redundant spirals defined by the disc, the less noticeable will be the adverse effect of a contaminating particle partially obstructing one or more of the spirals. As will be subsequently described, the illustrated embodiment utilizes four scanning spirals to obtain such scanning redundancy.
As shown in U.S. Pat. No. 3,011,020 issued Nov. 28, 1961 to G. M. Stamps, the coincident overlapping of the spirals forming the aperture with the elongated slit define a parallelogram-shaped aperture. During that finite time interval when the light passing through the aperture is sampled, the shape of the aperture will vary due to the continuous change in the included angle between one of the spirals and the elongated slit. Prior arrangements generally have integrated the light intensity reflected from the document during the sampling period, which of course, reduces system resolution. The illustrated embodiment increases system resolution by utilizing a segmented spiral in cooperation with a narrow elongated slit defining the exposure aperture. That is, the spiral is comprised of a series of contiguous, discrete, arcuate segments. Each arcuate segment of the spiral is maintained at a constant radius throughout its entire length with the light level passing through the aperture being sampled at approximately the middle of each segment. In this manner, the size and shape of the exposure aperture defined by the slit and the cooperating spiral is maintained constant throughout the sampling period with a resulting significant improvement in system resolution.